1. Field of the Invention
This invention relates to a fuel control device for an internal combustion engine, and specifically, transient control under forcible modulation control which oscillates air-fuel ratio between rich and lean values.
2. Description of the Related Art
As air-fuel ratio control in an engine (internal combustion engine), there is known forcible modulation control which oscillates combustion air-fuel ratio between rich and lean values with a predetermined amplitude, thereby forcibly oscillating exhaust air-fuel ratio.
Performing such forcible modulation control, for example when a catalytic converter provided in an exhaust passage of an engine is already activated, enables an improvement of exhaust clean-up performance by use of O2 storage function and HC storage function of the catalytic converter. Performing such forcible modulation control, for example when the engine is cold-started and the catalytic converter is not activated yet, immediately after the engine is started, in particular, enables promoted rise in catalyst temperature through forcibly-caused reduction reaction and oxidation reaction.
With an engine installed on a vehicle, there is, however, a problem that under such forcible modulation control, the engine manipulated to accelerate or decelerate in order to accelerate or decelerate the vehicle shows lowered transient responsiveness. Specifically, if the engine is manipulated to accelerate when the combustion air-fuel ratio is at a lean value, fuel correction cannot be made in time, which leads to a delay in increase in engine speed and torque, therefore, unsatisfactory acceleration and lowered drivability, and also to an increase in HC emissions due to misfire. On the other hand, if the engine is manipulated to decelerate when the combustion air-fuel ratio is at a rich value, fuel correction likewise cannot be made in time, which leads to excessive consumption of fuel, therefore, an increase in HC emissions and a decrease in gas mileage.
In this view, there has been developed a device which forbids the forcible modulation control when detecting the engine's being in an acceleration or deceleration state under the forcible modulation control (dither control) (see published Japanese Patent No. 2841806).
However, in the device disclosed in this publication, configured to forbid the forcible modulation control when detecting the engine's being in an acceleration or deceleration state under the forcible modulation control, if it takes time to determine that the engine is in an acceleration or deceleration state, it after all leads to bad transient responsiveness, thus, lowered drivability and gas mileage.
Further, the device disclosed in this publication forbids the forcible modulation control when detecting the engine's being in an acceleration or deceleration state under the forcible modulation control, which means that the forcible modulation control is not performed at all while the engine is in an acceleration or deceleration state, or in other words, in a transient state. This is, however, not desirable. For example, if the engine is manipulated to accelerate or decelerate while the forcible modulation control is being performed to raise the catalyst temperature immediately after cold start of the engine, the forcible modulation control is suspended while the engine is in an acceleration or deceleration state, so that the raising of the catalyst temperature is suspended, so that the activation of the catalyst converter is delayed.